I am soo mad right now ..i dont even know if its bad or if its okay to feel like this?

ok so everytime me and my boyfriend go out to the movies someone has to add up.. I really was looking foward to going alone as a couple ..and now he tells me that his sister and her boyfriend is coming too..is it ok to be upset cuz of this? or am i being rude..i just want time to be with eachother alone AS A DATE! but noooooo.